As known, the containers of the above mentioned type, after having been filled with hot—for example at about 85° C.—pourable products or liquids, are first subjected to a capping operation and then cooled so as to return to a room temperature. By effect of the capping operation, the heated air present in the top portion (“head space”) of the container expands causing a stress tending to produce a general swelling of the container at the side wall and at the base wall.
The following cooling to which the container is subjected, causes, vice versa, a reduction of the volume of air and minimally of the liquid product contained in the container; a depression is therefore created, which tends to pull the side walls and the base wall of the container inwards. This may determine deformations in the walls of the container if these are not rigid enough to resist the action of the above disclosed stresses.
In order to contain the depressive stresses generated during the cooling of the product within the containers without generating undesired deformations on the containers, they are typically provided, at the side wall, with a series of vertical panels, known as “vacuum panels”. These panels, in the presence of depressive stresses, are deformed inwardly of the container allowing it to resist to the hot fill process without generating undesired deformations in other areas of the container.
Likewise, the known containers intended to be subjected to a hot fill process can also have an optimised lower portion or base adapted to be deformed upwards under the action of the depressive stresses.
Even though the disclosed solutions allow to “relieve” the pressure stresses on specific parts of the containers, i.e. the vertical vacuum panels or the base, thus avoiding the occurrence of undesired deformations in other parts of the containers, they do not allow the cancellation of the above said stresses; in other words, the containers remain in any case subject to internal depressive stresses and must therefore be provided with a structure capable of resisting such stresses.
Patent application WO2006/068511 shows a container having a deformable base, which can have two different configurations: a first unstable configuration, in which this base has a central area projecting downwards with respect to the outermost annular area immediately adjacent thereto, and a second stable configuration, in which the central area is retracted inwardly of the container, i.e. it is arranged in a higher position with respect to the adjacent annular area.
Following the filling with the hot pourable product, the base of the container has the first unstable configuration and must be supported by a special cup element to which it is coupled. Thereby, the downward deformation of the base of the container can be maximised without compromising the stable support of the container, since such a support is provided by the cup element. Following the cooling, the base can be displaced by an external action, for example a vertical thrust upwards performed by a rod or plunger, in the second stable configuration with the subsequent possibility of removing the cup element.
The displacement of the base of the container from the first to the second configuration determines a considerable reduction of the containment volume of the container, much higher than would be obtained in the known containers simply by the deformation of the base by the effect of the sole depressive stresses; the final effect is therefore substantially the cancellation of the depressive stresses acting on the inside of the container.
The applicant has observed that this kind of operation may become quite critical, in particular when the time necessary to perform the deformation of the base of the container has to be strongly limited or reduced, for instance due to production constraints; in such cases, the plastic material may return at least in part towards the original first configuration after release of the plunger; this normally occurs when the plastic material has a reaction time exceeding the time for performing the operation of deformation.
The non-correctly formed containers have therefore to be rejected at the end of the production line.
Another problem posed in connection with the described containers is the complexity of the plant layout for producing them. In particular, the disclosed containers must be subjected to the following operations to achieve their final shape:                a filling operation with the hot pourable product on a filling machine;        a subsequent operation of capping on a capping machine;        a cooling operation in an appropriate station;        an inversion operation on a relative processing machine, in which the bases of the containers are mechanically displaced from the first to the second configuration;        a labelling operation on a relative labelling machine; and        possible further finishing operations if required.        
As it is known, the filling machines, the capping machines and the labelling machines are generally rotating machines, in which the containers are fed on respective carousels. In particular, each carousel is provided with a plurality of operative units for receiving and processing the containers, uniformly distributed about the rotation axis of the carousel; more precisely, each operative unit is commonly provided with an element for supporting the relative container which maintains it in a predetermined position for carrying out the specific operation/s.
As can be easily noted, the process for the production of the above said finished containers is rather time-consuming and requires considerable room within the relative plants; in order to carry out the different operations indicated, it is necessary to provide a relatively high number of machines and conveyors adapted to transfer the containers from a machine to another.
A further problem posed in connection with the above-described containers is the correct application of the labels on the designated surfaces of such containers. In particular, in order to be applied in a correct way, a label requires a receiving surface having a well-defined geometry as well as a sufficient rigidity. This second feature of the receiving surface is particularly important for self-stick labels or pressure-sensitive labels.